In Long Term Evolution (LTE), the scheduler is placed in the eNodeB and the Medium Access Control (MAC) layer. The scheduler assigns radio resources, also called Resource Blocks (RB), for the downlink (assignments) as well as for the uplink (grants) using the Physical Downlink Control Channel (PDCCH). Also, information concerning which transport format to use is comprised within the assignment and grant, respectively.
The radio downlink is the transmission path from a base station, e.g. an eNodeB to a terminal, or a User Equipment (UE) as the terminal also may be referred to as. The uplink is the inverse of a downlink, i.e. the transmission path from the terminal to the base station.
The terminal supplies the eNodeB with information about the data in its buffers using two mechanisms; a 1-bit scheduling request (SR) or buffer status reports (BSR). Buffer status reports are transmitted on a data channel such as Physical Uplink Shared Channel (PUSCH) mostly together with user data. Before access to the data channel is granted, scheduling requests are transmitted on a control channel such as e.g. Physical Uplink Control Channel (PUCCH) or Random Access Channel (RACH). If the terminal has a valid PUCCH resource for scheduling request configured in any transmission time interval (TTI) it sends a one bit scheduling request when the timing is right. Otherwise it initiates a random access procedure and cancels all pending scheduling requests. This process is illustrated in FIG. 1, which depicts prior art uplink scheduling.
The terminal is only allowed to use the PUCCH at predefined points in time determined by the Dedicated Scheduling Request (D-SR) interval. The delay between the actual generation time of a data packet such as e.g. a Voice over the Internet Protocol (VoIP) packet and the sending of the D-SR can thus become as large as the D-SR interval.
In the present context, the generation time is defined as the actual time when the data was put in the transmit buffer at the terminal and arrival time is defined as the time when the eNodeB receives the scheduling request.
If the network traffic within the wireless system consists mostly of VoIP packets, there will be many active terminals. Each terminal will get an uplink grant with an interval of around Z ms, where Z depends on the number of terminals. Until a terminal with data in its buffer gets a grant it will continue sending D-SRs on the PUCCH each time it can. Thus, at high load there will be many terminals which are actively using their D-SR resources. Although, there may be sufficient codes to assign to the terminals, the interference will rise when many users transmit simultaneously setting an upper limit to how may users that can actively send to maintain a low error rate on the PUCCH Scheduling Request resource. Therefore, increasing the D-SR interval to keep the PUCCH load reasonable may be necessary when there are many VoIP users. However, the larger the D-SR interval is, the less precise knowledge will be given to the eNodeB concerning the generation time of data in the terminal buffer.
When a voice user is listening instead of talking, the terminal sends Silence Insertion Descriptor (SID) frames with much larger spacing then the voice frames. Thus a smaller D-SR interval is possible as the terminal will need to send a D-SR less often.
When using service aware buffer estimation such as e.g. VoIP aware buffer estimation, just to mention an arbitrary example, the generation time is valuable. The VoIP aware buffer estimation algorithm moves between two states, SID and TALK and a state change should preferably occur when the codec switches between the corresponding states. The TALK state is a proactive buffer estimation state which guesses when the next voice frame will arrive and which size it will have, while the SID state is a passive state that expects Scheduling Requests when data has arrived for a user.
As voice frames arrive every 20 ms to the terminal buffer using for instance Adaptive Multi Rate (AMR), the better the algorithm knows the generation time, the more exact it will predict the buffer size.
The larger the D-SR interval is, the larger the difference between the generation time of the VoIP packet and the arrival time noted by the eNodeB may be. This makes it harder to accurately predict the buffer state and schedule delay-sensitive services, increasing the need for explicit signalling and decreasing the efficiency of the uplink assignments.
The VoIP aware buffer estimator use the arrival of the D-SR, though processing time is deducted, as the VoIP packet generation time, and this can be very different from the actual VoIP packet generation time. Furthermore, the eNodeB scheduler does not have correct packet delay information and may schedule the VoIP packet too late, especially in scenarios where the required VoIP delay is relatively short.